1. Technical Field
This invention relates to a moving-coil electrodynamic motor, and more particularly, to a moving-coil electrodynamic motor for a loudspeaker. This invention also relates to a loudspeaker and a pole piece.
2. Related Art
To limit harmonic distortion on reproduction, loudspeakers should have good response linearity. Linearity is obtained particularly when a coil intercepts a homogeneous magnetic field flux during movement of the coil. The distance of a maximum linear displacement of a coil may be referred to as a maximum linear excursion or Xmax, which can be abbreviated to XM. Linearity also may be obtained by at least two other methods. One method involves making the coil a homogeneous winding with a considerable height (along the front-rear axis of the coil movement) greater than the height of the gap where the coil is disposed. In this way, as long as a coil remains entirely in the gap, force and current flowing in the coil remain proportional. This configuration, known as a long coil configuration, is suitable for boomers. The second method relates to reducing the height of a coil relative to the height of a gap. This configuration, known as a short coil configuration, is often used for tweeters and may be used for mediums.
Despite improvements to reproduce characteristics of loudspeakers having a moving-coil electrodynamic motor, asymmetry of magnetic field toward both ends of a gap undermines linearity and eventually affects the maximum linear excursion of a coil. In addition, loudspeakers have sources of sound distortion, such as complex electromagnetic phenomena created by variable electric fields, such as displacement of conductors in the magnetic field, modulation of the static magnetic field of a magnet in a gap by variable current flowing a coil, and a coil “DC shift” and generation of Foucault current. Accordingly, there is a need for a loudspeaker system that overcomes the foregoing drawbacks.